1. Field of the Invention
The present invention relates generally to an apparatus and method for providing a packet data service in a communication system, and in particular, to an apparatus and method for transmitting and receiving a PDCCH (Packet Data Control Channel) to detect a packet data transmission and reception scheme.
2. Description of the Related Art
While a typical mobile communication system supports voice service only, user demands and the development of mobile communication technology have developed a mobile communication system that additionally supports data service.
In a mobile communication system supporting multimedia service including voice and data service, a plurality of users receive voice and data service in the same frequency band. To do so, TDM (Time Division Multiplexing) or CDM (Time Division Multiplexing/Code Division Multiplexing) is supported for data transmission. These transmission schemes are not sufficient to provide service to many users. Thus there is a need for exploring a method of accommodating more service users.
The mobile communication system uses a PDCH (Packet Data Channel) and a PDCCH. The PDCH delivers packet data on a PLP (Physical Layer Packet) basis and the PDCCH delivers control information about the PDCH. The PDCCH is also used to detect the transmission and reception scheme of packet data transmitted on the PDCH.
FIG. 1 is a block diagram of a conventional PDCCH transmitter when only TDM is supported for PDCH transmission.
Referring to FIG. 1, it is assumed that control information transmitted on the PDCCH, a PDCCH input sequence is 18 bits though it is not limited to 18 bits.
The PDCCH input sequence includes a 6-bit MAC (Medium Access Control) identifier (ID), a 2-bit SPID (Sub-Packet ID), a 2-bit ARQ (Automatic Repeat Request) ID, a 3-bit payload size, and a 5-bit Walsh space indicator (WSI). The MAC ID is assigned to a user that is to receive a high-speed packet data service during system access in order to identify the user.
In general, a high-speed packet data transport channel delivers data on a sub-packet basis in a mobile communication system supporting high-speed packet transmission. The SPID identifies a sub-packet to support retransmission. The ARQ ID identifies a parallel transport channel to support continuous data transmission to one user. The payload size is the number of bits in one sub-packet. The Walsh space indicator is an information indicating Walsh codes used for the PDCH.
At system access, all MSs (Mobile Stations) that are to receive the high-speed packet data service are assigned MAC IDs from a BS (Base Station). Each time the MSs receive a PDCCH, they demodulate the PDCCH and determine whether a packet is destined for them. If the packet is destined for a particular MS, the MS demodulates a PDCH using the control information of payload size, SPID, ARQ ID, a Walsh space indicator on the PDCCH.
In operation, a CRC adder 101 adds eight CRC bits to the 18-bit PDCCH input sequence to allow detection of errors in the control information. As the number of CRC bits increases, transmission error detection performance increases.
A tail bit adder 102 adds eight tail bits with all 0s to the CRC-attached 26-bit control information received from the CRC adder 101. A convolutional encoder 103 encodes the output of the tail bit adder 102 at a code rate of ½.
A puncturer 104 punctures 20 symbols in the code symbols received from the convolutional encoder 103 to minimize performance degradation and match to a desired data rate. An interleaver 105 interleaves the punctured symbols to permute the sequence of the symbols and thus reduce burst error rate. A modulator 106 modulates the interleaved symbols in a modulation scheme such as QPSK (Quadrature Phase Shift Keying).
FIG. 2 is a block diagram of a conventional PDCCH receiver when only TDM is supported for data transmission. Referring to FIG. 2, data received from the PDCCH transmitter illustrated in FIG. 1 on a radio channel is converted to transmission symbols through frequency down-conversion. Then, a deinterleaver 201 deinterleaves the symbols, a depuncturer 202 depunctures the deinterleaved symbols, and a convolutional decoder 203 decodes the depunctured symbols. Then a CRC checker 204 CRC-checks the decoded symbols to determine whether the received data has errors. A packet data control information detector 205 detects 18-bit packet data control information when the received data turns out good by the CRC check and ends the reception operation when it turns out failed.
As described above, the conventional PDCCH supports only TDM transmission of the PDCH. Due to the demand for accommodating more users, a system supporting TDM and CDM simultaneously (TDM/CDM) for data transmission on the PDCH has been suggested. In such a system, different Walsh codes are assigned to a plurality of users for the same time period, for data transmission. Thus, the conventional transmitter and receiver are not suitable for the system because they cannot identify users and transmit data accurately.